Conventional fire and smoke detection methods and apparatus generally include the use of well-known smoke and heat detectors, such as ionization smoke detectors and photooptical smoke detectors. These devices are used as independent detector systems, such as in home use, or as peripheral devices reporting alarm conditions to in a centralized system as is commonly used in larger buildings and industrial use.
Whether these devices are used as stand alone systems or peripheral devices, the principle of their operation is generally the same. For example, a light-scattering type photooptical detector generally comprises a light emitting source, such as light-emitting diode (LED), and a light sensor, such as a photo diode, contained in a substantially light proof sample chamber having low reflectance walls. Light from the light-emitting source is reflected off the low reflectance walls to the light sensor, which is out of the direct path of light. Air surrounding the photooptical detector passes generally freely in and out of the sample chamber. When ambient air is relatively free from fire or combustion products, such as smoke, only a relatively small amount of light from the LED is reflected off the chamber walls to be received by the light sensor. This low light receiving condition is the normal or no-alarm state in the photooptical detector.
As the amount of combustion products increases, the amount of light reflected or scattered by the combustion products increases. The increased light scattering generally increases the amount of light reaching the light sensor proportionally. This phenomenon generally correlates to percent obscuration per foot which is defined by Underwriters Laboratories, Inc. (UL) Standard 268 (May 2, 1989). A simple explanation of percent obscuration per foot is the reduction in visibility the human eye would see in a room containing combustion products.
The amount of light detected by the light sensor may be represented as a generally steady direct current voltage output, such as between 0 Vdc and 5 Vdc, for example. This may be illustrated as the generally flat curve labeled "Detector Voltage" in FIG. 1a. As the amount of light detected by the light detector increases due to increased combustion products, the voltage output generally increases. Conventional ionization detectors also output increasing voltage as the smoke condition rises. When the detector voltage output reaches a predetermined threshold, (illustrated as the flat curve labeled "Alarm Threshold" in FIG. 1a) an alarm condition is indicated by audible, visual or other indications for appropriate investigation or evacuation of the alarm area.
While this method is recognized as being generally effective, the problem encountered is that such a device generally becomes dirty with age and upon exposure to combustion products, dust and other film-forming contaminants. Thus, photooptical detectors must be cleaned or replaced periodically, especially after exposure to combustion products. Until cleaning or replacement of the detector unit can be made, the sensitivity of these conventional devices is adversely affected. Thus, for example, as the amount of contaminants increases and collects on the chamber walls of the photooptical detector, the amount of light perceived to be transmitted increases and the voltage output alters to indicate increased percent obscuration per foot. This perceived increase in percent obscuration per foot will exist in a dirty photooptical detector even when the air is substantially free from all combustion products.
Similar contamination problems occur in ion detectors. In addition to dust and film formation, other factors affecting detectors include humidity, altitude (ionization, especially), wind velocity (ionization, especially), voltage supply variations, detector component tolerances and component aging.
The sensitivity of conventional devices may be measured as the difference between the voltage output at substantially combustion free ambient conditions and the predetermined alarm threshold of the individual detector. For example, a low sensitivity system may be seen in FIG. 1b, where the difference between the Detector Voltage and the Alarm Threshold is relatively large. High sensitivity is illustrated in FIG. 1c, where the difference between the Detector Voltage and the Alarm Threshold is relatively small. Thus, as the voltage output increases due to accumulated dirt and combustion products, the sensitivity increases where the alarm threshold remains the same. Therefore, the addition of even small, transient combustion products or dust particles, increasing light scattering, when combined with increased sensitivity of the dirty detector, often results in false alarms.
Because the threshold voltage of conventional detectors is typically set via a potentiometer and resistor divider network in the detector head, the sensitivity of a given detector may be adjusted by manually adjusting the alarm threshold up or down to achieve the desired sensitivity. Desired sensitivity may also be restored in photooptical detectors, for example, by cleaning the chamber and LED and light sensor components, thereby reducing the detector voltage output. However, these adjustments require that a skilled technician disassemble the detector at its location which is both inconvenient and non-cost effective. Moreover, these adjustments are only temporary and must be performed periodically. In addition, such periodic adjustments do not compensate for the day-to-day unpredictable changes in dust and other contaminant accumulation in the detector.
In view of the deficiencies and inefficiencies of the prior art, it would desirable to have an alarm detection system which recognizes and compensates for changes in detector sensitivity due to various factors on a real-time basis.